Arrangement of a valve stack for high voltage direct current in a valve hall

ABSTRACT

The invention relates to a new arrangement of at least one valve stack (2) for high voltage direct current in a valve hall (1). Said valve stack (2) has a voltage to ground which, in operation, increases along the stack (2). The invention comprises arranging the stack (2) in a substantially lying position at such a distance from the floor and roof (9), respectively, of the valve hall (1) that the smallest electrical flashover distance between live parts on the stack (2) and said floor and roof (9), respectively, is at least contained. Further, the electrical connection is arranged via bushings through the roof (9) of the valve hall (1).

It is known that in a valve hall for generation of high voltage directcurrent, the elongated stacks of rectifier valves may be positionedvertically in an upright or suspended manner. So far, this has beenconsidered to lead to the lowest cost for valves and valve hall.However, for economic reasons, the necessary bushings must be placed inthe walls of the hall in such an arrangement. The bushings will thenhave an almost horizontal extension and will be positioned, at leastpartially, in the rain shadow of the roof of the valve hall. In case ofdirect voltage levels in excess of about 400 kV, this may lead toflashover caused by dirt, which is due, in part, to that part of theinsulator of the bushing, which lies in the rain shadow, not beingcleaned by precipitation, and, in part, to the lower side of thehorizontal insulator also being protected against rain and consequentlyrisking a continuous "stripe of dirt" on the surface. To prevent this,it has hitherto been necessary to grease the bushings--an action whichhas been looked upon with great disapproval by the plant owner since ithas constituted a heavy part of the maintenance of the valve hall andhas made a high voltage direct current plant appear particularlymaintenance-demanding in comparison with an alternating current plant.

It is known from experience that insulators which are placed verticallyoutside the rain shadow run a very small risk of flashover up to adirect voltage of at least 600 kV. Thus, it should be a suitablesolution to place all the bushings on the roof, but in knownarrangements of valve stacks this infallibly leads to a considerablylarger valve hall with an ensuing higher total cost.

According to the invention, there is now proposed a new method ofplacing the valve stacks in the valve hall. This method permits thebushings to extend through the roof without the volume of the valve hallhaving to be increased. The invention comprises arranging each valvestack, which is composed of a number of electrically series-connectedvalve modules arranged one after the other and which in operation have avoltage to ground which increases along the stack, in a substantiallylying position. The valve stack is to lie at such a distance from thefloor and roof of the valve hall, respectively, that the smallestelectrical flashover distance between live parts on the stack and thefloor and roof, respectively, is contained and that electricalconnection to the stack is arranged at bushings through the roof of thevalve hall. In principle, this means that the entire valve hall isoverturned, so to speak, and, contrary to prior art arrangements, itsvolume need not be increased in spite of the fact that all thebushings--both for alternating and direct voltage--have now beenarranged through the roof.

According to a special alternative, the volume of the valve hall mayeven be reduced if the roof of the valve hall is made sloping towardsthe horizontal plane and this is combined with the fact that theflashover voltage to ground increases along the valve stack from one endthereof to the other.

The volume of the hall may be further reduced if the valve stack issuspended in such a way that the distance between live points on thevalve stack and the roof and floor, respectively, of the valve hall isonly kept somewhat greater than the current flashover distance betweenthe corresponding points and the roof and floor, respectively, of thevalve hall.

A further simplification of the erection may be made if three valvestacks in a valve hall are arranged in a star- or Y-configuration withthe high voltage ends of the valve stacks brought together andelectrically interconnected at the center of the star and connected to acommon bushing for the high voltage direct current. In addition to thisembodiment providing a saving of bushings, the volume of the hall may befurther reduced by the roof now being given a dome-like shape whichmakes it possible to make all the walls in the hall lower in comparisonwith an arrangement in which the stacks are suspended in a lying, butparallel, manner. In the latter case, the whole wall of the hall againstthe high voltage end of the stacks must be made considerably higher thanthe corresponding wall at the low voltage end.

The invention will be best understood with reference to the accompanyingdrawings, in which

FIG. 1 shows a side view of the valve hall which is commonly used today,

FIG. 2 shows a proposal for a new arrangement of the valve stacks in avalve hall. FIG. 2 is a section of A--A in FIG. 3,

FIG. 3 shows a section of B--B in FIG. 2,

FIG. 4 shows the valve hall according to FIGS. 2 and 3 seen from abovewith transformers, not shown in FIGS. 2 and 3,

FIG. 5 shows a wiring diagram for the rectifier station according toFIG. 4,

FIG. 6 shows an embodiment with a sloping roof of the valve hall,

FIG. 7 shows a combination of a sloping roof and an obliquely suspendedvalve stack,

FIG. 8 shows the principle of a star-shaped arrangement of lying valvestacks, and

FIG. 9 shows a possible embodiment of a valve hall for such an erectionor suspension of the valve stacks.

In FIG. 1, 1 designates the valve hall and 2 a valve stack for highvoltage direct current suspended therein. The valve stack 2 is composedof electrically series-connected valve modules 3 which are bridged byovervoltage protective means 4. The valve stack 2 is supplied from atransformer (not shown) via the bushings 5 and 6. The bushing 7 isintended for connection to a line or cable for high voltage directcurrent whereas the low voltage side of the valve stack 2 is intended tobe connected, via the bushing 8, to the electrode (not shown) of thedirect current transmission. The valve stack 2 is suspended from theroof 9 of the valve hall by means of suspension insulators 10. In thistype of valve halls, thus, the risk of flashover caused by dirt acrossthe bushing 7 is great at direct voltage levels above 400 kV. This isdue to the fact that, especially in case of certain wind directions, thebushing 7 comes within the rain shadow of the high building.

With regard to FIG. 2, it may be said that, in principle, the valve hall1 and the valve stack 2 have been overturned so that both of them cannow be said to be lying down instead of standing on edge. For the sakeof simplicity, the objects in FIG. 2 have been given the samedesignations as those in FIG. 1. By the lying arrangement of the valvestack 2, the bushings 5-8 may be placed vertically on the roof 9 of thevalve hall 1 without the volume of the valve hall 1 having to beincreased and without the total cost of the plant increasing.

FIG. 3 shows, as mentioned, a section B--B of FIG. 2. In FIG. 3 the samedesignations as before are still used. FIG. 3 shows two additional valvestacks 2' and 2" which together with the valve stack 2 form a completerectifier station. This is supplied with alternating current from thetransformer 11, the high voltage side of which has been connected to thebushings 6 according to FIG. 2. These bushings 6 are in FIG. 3 concealedby the bushings 7.

FIG. 4 shows the same valve hall 1, seen from above. In FIG. 4, 12designates a additional transformer which supplies the valve stacks 2,2' and 2" via the bushings 5. The line 13 leads to a line for the highvoltage direct current transmission, whereas the line 14 leads to anelectrode for the corresponding transmission.

FIG. 5 shows an electric wiring diagram for a plant according to FIG. 4.

FIG. 6 shows a valve hall 15 with a sloping roof over the suspendedvalve stack 2. In this arrangement of the valve stack 2 the circumstancehas been utilized that the voltage at one end of the valve stack 2 islow and then increases to high voltage at the other end of the stack 2.Consequently, the distance to the roof 16, i.e. to ground, may beallowed to be smaller at the low voltage end of the valve stack 2 thanat its high voltage end. The volume of the valve hall 15 may thus bereduced in relation to the embodiment described above.

FIG. 7 shows another possible modification. Here, it has been possibleto make the roof 17 of the valve hall 15 with an even greater slope thanin FIG. 6. This is due to the fact that, in suspending the stack 2,account has been taken of the fact that a lower distance between the lowvoltage end of the valve stack 2 and the floor of the valve hall, inrelation to the corresponding necessary distance at the high voltageend, can also be allowed. As will be clear from FIG. 7, this means thatthe valve stack 2 is arranged such that the distance between live pointson the valve stack 2 and the roof 17 and floor 18, respectively, of thevalve hall 15 is kept somewhat greater than the current flashoverdistance between the corresponding points and the roof 17 and floor 18,respectively.

FIG. 8 shows the principle of a star-shaped arrangement of the valvestacks 2, 2' and 2" in a valve hall 19, The valve stacks are herearranged with their high voltage ends converging at the center of thestar. This means that a common bushing for the high voltage directcurrent may be used and that the largest necessary insulation distanceto ground for the three valve stacks will be in common.

If this is taken into consideration, the valve hall 19 mayadvantageously be given the shape as shown in FIG. 9. As will be clearfrom this FIG. the valve hall 19 has here a dome-like shape with longside walls all around as a consequence of the low voltage ends of thestacks 2 facing the wall of the hall whereas their high voltage ends, soto speak, share the insulation distance from the center of the star tothe highest point of the domed roof. If, in addition, the stacks were tobe suspended as shown in FIG. 7, it would be possible to make the sidewalls even lower, thus saving additional valve hall volume.

We claim:
 1. An arrangement of at least one elongated valve stack (2)for high voltage direct current in a valve hall (1), which said valvestack (2) is composed of a number of electrically series-connected valvemodules (3) arranged one after the other, said valve stack (2) beingprovided with d.c. terminals (13, 14) at the ends of the valve stack (2)and at least one a.c. terminal between said ends and, in operation,having a voltage to ground which increases along the stack (2), whereinthe stack (2) is arranged in a substantially lying position and at sucha distance from the floor and roof (9), respectively, of the valve hall(1) that the smallest electrical flashover distance between live partson the valve stack (2) and said floor and roof (9), respectively, iscontained, and that electrical connection to the valve stack, (2) isarranged via bushings through the roof (9) of the valve hall (1).
 2. Anarrangement according to claim 1, wherein the valve stack (2) isarranged suspended from the roof (9) of the valve hall (1) by means ofsuspension insulators (10).
 3. An arrangement according to claim 1,wherein at least that part of the roof (9) of the valve hall (1) whichlies directly over a valve stack (2) is made sloping towards thehorizontal plane.
 4. An arrangement according to claim 3, wherein thedistance between live points on the valve stack (2) and the roof (9) andfloor, respectively, of the valve hall (1) is somewhat greater than thecurrent flashover distance between the corresponding points and saidroof (9) and floor, respectively.
 5. An arrangement according to claim1, including three valve stacks (2) arranged in a Y-configuration withthe high voltage ends of the stacks (2) interconnected at the node ofthe Y and connected to a common high voltage bushing.